Fluoropolymers are long-chain polymers comprising mainly ethylenic linear repeating units in which some or all of the hydrogen atoms are replaced with fluorine. Examples include poly(tetrafluoroethylene) (PTFE), perfluoromethyl vinyl ether (MFA), fluoro ethylene propylene (FEP), perfluoro alkoxy (PFA), poly(chlorotrifluoroethylene) (PCTFE) and poly(vinylfluoride) (PVF). They are amongst the most chemically inert of all polymers and are characterized by an unusual resistance to acids, bases and solvents. They have unusually low frictional properties and have the ability to withstand extremes of temperature. Accordingly, fluoropolymers are utilized in a wide variety of applications in which resistance to extreme environments is necessary. Current applications include the formation of tubing and packing materials within chemical plants, semiconductor equipment, automotive parts and structural cladding.
There are several application methods, one of which requires the powdered form of the fluoropolymer. Here the fluoropolymer is typically applied to a surface by electrostatic spraying of the powder. Uses include the coating of household cookware to increase non-stick properties and abrasion resistance, and the coating of automotive parts to increase resistance to environmental weathering.
At present, two methods are used to produce the powdered form of a fluoropolymer. Spray drying methods comprise the pumping of an aqueous dispersion of the fluoropolymer feed into an atomizing system, generally located at the top of a drying chamber. The liquid is atomized into a stream of heated gas to evaporate the water and produce a dry powder. This method has several limitations. The requirement that the aqueous dispersion is pumped into the atomizing system limits the use of this process to pumpable materials, and the spray dried agglomerates are tightly bound to each other and resist subsequent disagglomeration. In addition, only non-fibrillatable materials can be processed, as atomization may result in the fibrillation of the fluoropolymer, resulting in an intractable ‘marshmallow’ material which is difficult to handle.
An alternative method involves the coagulation of the particles within an aqueous dispersion. Coagulation is facilitated by the use of high mechanical shear, the addition of acids or the addition of gelling agents and subsequent treatment with a water immiscible organic liquid. The coagulated particles can be separated from the residual liquid by filtration and subsequently dried, typically using tray, belt or flash dryers. The coagulated granules are usually case hardened for ease of handling. However, the formation of agglomerates results in a particle size that is too large for use in conventional powder spray application techniques. Milling, traditionally used to adjust the particle size distribution, can cause fibrillation of the particles, to produce an intractable material which is difficult to handle. The case hardened material also produces a tight agglomerate which resists subsequent disagglomeration.